16 - Climate change and cropping in Western Australia

Ian Foster and Imma Farre, Department of Primary Industry and Regional Development

Almost 30 years ago, the Bureau of Meteorology presented its projections of future climate for the south-west of Western Australia. Their most likely climate scenario was –

a southward shift in winter weather systems;

an expansion or intensification of the tropics;

Winter rainfall was projected to decrease, while summer rainfall would increase;

Mean temperatures would also increase over time.

Observations of climate since then have confirmed the accuracy of their projection, with winter rainfall declining faster than expected and mean temperatures increasing, though not as strongly as originally expected.

In more recent years, CSIRO and the Intergovernmental Panel on Climate Change (IPCC) have provided similar climate projections for the south-west of WA. These projections, if accurate like the BOM projections, have significant implications for Western Australian grain production, which greatly depends on rainfall between May and October. (Editor - For more information on this point, refer to last week's post by Dr David Stephens)

If these projections are carefully downscaled using global climate models (GCMs), it is possible to gauge how future climate may potentially affect grain yield across the grain-belt of Western Australia.

In general, future winter growing seasons are likely to be drier and warmer, with growing season rainfall levels around 10% less than now, and summer rainfall around 6% higher. In general, rainfall will remain highly variable, as is the case now.

Applying the downscaled climate data to the APSIM-Wheat crop model, wheat yields on three different soil types, at scores of locations, can be examined. APSIM-Wheat simulates daily values of root growth, biomass and grain yield based on information on daily weather, soil type and crop management. It calculates the water-limited potential yield of the site (that is, the yield not limited by weeds, pests, and diseases, but limited only by temperature, solar radiation, water, and nitrogen supply at that site).

This wheat model was run at 27 locations across the WA grain-belt using two sets of climate data for 30-year periods for current and future climate -

Current simulated climate is for the period 1975-2004 with a current level of CO2 (350 ppm).

Future simulated climate is for the period 2035-2064 with higher CO2 levels (440 ppm).

Future mean wheat yields for 2035-2064 were about 10% lower across all locations than simulated yields for current conditions, with differing responses according to soil type and location.

Heavy clay soils in low rainfall areas are most at risk from the projected change in climate (showing a possible yield decline of some 20%), however there are potential increases in yields on duplex soils in high-rainfall regions (mean yields could be higher by about 10%).

There is an increased risk of low yields (less than 1 t/ha) in low rainfall areas on all soil types - especially for heavy soil. The frequency of occurrence of high yields decreased at low rainfall locations.

Figure 1. Percentage change in average simulated wheat yields for 2035-2064, compared with yields for 1975-2004. Note: no allowance is made for technology change in coming years. Responses have been mapped to dominant soil types by Dennis Val Gool, DPIRD.

In high-rainfall areas, the incidence of high yields increased markedly under future climate. This is likely the result of decreased waterlogging and nutrient loss, as well as higher temperatures and CO2 concentration.

Figure 1 shows percentage changes in average wheat yield under future climate, compared with yields under current climate. Yield responses have been mapped according to the main soil type at each location.